1. Field of the Invention
The present invention relates to a sensitizing agent for electroless plating and to a method for sensitizing substrates with the agent.
2. Description of Related Art
Electroless plating, including electroless copper plating, is widely applied in the field of printed circuit boards to form circuits on electrically insulative substrates such as plastic laminated boards and the like. In this case, the substrate to be electroless-plated is subjected to sensitization, i.e. treatment to form catalytic nuclei for electroless plating thereon, before immersing in an electroless plating bath. For instance, copper ions in the bath are deposited as metallic copper around these catalytic nuclei and grow into copper films, thus forming circuits. To sensitize electrically insulative substrates, fine particles of metallic palladium, silver, copper, or nickel which act as catalytic nuclei for electroless plating are formed on the surfaces of the substrates. As an example, palladium particles for this purpose, according to the prior art, are produced by such reaction as follows: ##STR1## PdCl.sub.2 --SnCl.sub.2 --HCl sensitizing systems are described in U.S. Pat. Nos. 3,011,920, 3,960,573, and 3,969,554.
The PdCl.sub.2 --SnCl.sub.2 --HCl sensitizing system containing much impurities including Sn and Cl is strongly acidic and, therefore, apparatuses exposed thereto are damaged and the pink-ring phenomenon of inner layer circuits in multilayer circuit boards occurs.
The pink-ring phenomenon of inner layer circuits is described as follows:
Multilayer printed circuit boards are produced in the manner that circuits are formed by etching copper-clad laminated sheets, the resulting inner layer circuit sheets and prepregs (products of impregnating base materials such as paper and glass fabrics with thermosetting resins, followed by drying them) are superposed alternately one over another; the top layer is overlayed with a copper foil or a laminate clad with copper on one side; the whole laminate is heated and pressed into a single body; the copper on the top layer is processed to form a circuit correspondingly to the pattern of the inner layer circuit; through-holes are formed by perforating layers at required positions (including a part of the inner layer circuits); and a metal layer is formed on the inner wall of each through-hole by electroless plating. In this process, the surfaces of the inner layer circuits are coated with copper oxide (usually 1-3 .mu.m thick) so as to enhance the adhesion between each inner layer circuit and the prepreg facing it. The inner wall of each through-hole is sensitized before the electroless plating. When the PdCl.sub.2 --SnCl.sub.2 --HCl sensitizing system is used for the sensitization, portions of the oxide coat exposed to the through-holes will dissolve in hydrochloric acid, and hydrochloric acid will diffuse into the interface between the surface of the inner layer circuit and the prepreg resin layer, thus causing delamination. This is the pink-ring phenomenon. When this phenomenon occurs the bond strength and the insulation resistance of the laminate will become lower.